For this reason alone, Betfair has no reason to ban or close your account when you should win. This is what Betfair first became famous for; peer-to-peer betting. Whilst the Exchange account won't be closed for winning, there is an additional commission rate.
Bonuses can only be used for non-Exchange sports bets. EXCHANGE BETS EXCLUDED. If you make more than one selection in your betslip which would comply with the terms of a bonus, then the total amount of these will be deducted from the bonus balance.